The present invention relates generally to safety pressure relief devices and more particularly to reverse buckling rupture discs which are designed to reverse and rupture at low bursting pressures and are highly reliable.
Relief devices of the type commonly known as rupture discs have been utilized in industry for many years to provide a safety mechanism to relieve excess pressure from an overpressurized system or vessel in a reliable manner. The rupture disc is most frequently placed in a vent or a pressure vessel or the like so as to prevent flow of fluid through the vent until the disc ruptures. Through the years, numerous improvements have been made in the rupture disc concept in order to reduce the cost and improve the reliability of the disc.
A specific type of disc normally referred to as a reverse buckling rupture disc has also been utilized for a number of years and functions under the principle that a disc dome is formed in the disc which is positioned in the vent such that the disc dome points toward or faces the pressure side of the vent, i.e., the convex side of the disc dome faces the internal region or upstream side of the vent where pressurized fluid is likely to produce an overpressure that would be dangerous or destructive if not relived. One advantage of reverse buckling type discs is that systems being protected by the discs can be operated at pressures relatively close to the bursting pressure of the disc without producing fatigue and failure which occurs in many forward acting bursting discs when operated for long periods of time near the rated bursting pressure of such devices. When fluid pressure reaches a preselected pressure for which the disc dome was designed to rupture, the disc dome starts the collapse, i.e, the column or arch of the disc dome on one side thereof starts to buckle. It is believed that as the arch on one side of the disc dome starts to collapse, a buckling-type wave typically propagates across the surface of the disc dome to the opposite side of the disc dome where total collapse eventually occurs. This buckling wave tends to create a whiplash effect on the opposite side of the disc dome so that the disc dome at this location is rather violently urged in the direction to which the concave region of the disc dome faces (i.e., the downstream side of the vent).
One disadvantage of some conventional reverse rupture discs is that if they are damaged during handling, installation, or otherwise, they can buckle at a pressure below the rated pressure for the disc. In some cases, the disc will buckle at a pressure of between 40 and 80 percent of the rated pressure. For example, if the rated pressure is 100 psi, a damaged disc may buckle at between 40 and 80 psi. While the reverse rupture disc may buckle or reverse at such pressures, it will not necessarily open at these pressures and once a reverse rupture disc becomes inverted, it thus acts as a forward acting rupture disc which will rupture at a higher pressure than the initial buckling pressure. For such discs the rupture pressure may be as much as three to ten times the rated pressure.
Another disadvantage of some conventional reverse rupture buckling devices is that they are incapable of reversing and rupturing at low bursting pressures. Bursting pressures are generally defined relative to the size of the disc. For example, 15 psig would be a low bursting pressure for a 2 inch diameter disc made of stainless steel. Conventional limitations to achieving low bursting pressures have been twofold (1) causing the rupture disc dome to reverse at a low pressure, and (2) being able to open the rupture disc at the lower reversal pressures. As previously mentioned, some damaged, conventional reverse buckling rupture discs may reverse at a low pressure but not rupture at that pressure. Also, it is more difficult to rupture conventional reverse buckling rupture discs at low pressures where the media is noncompressible (e.g., a liquid). This is because a noncompressible media such as a liquid does not impart the same dynamic energy to the dome during collapsing as a compressible media does.
Many of the conventional reverse buckling rupture discs include knife blades positioned on the concave side of the disc dome which are normally in spaced relationship to the disc dome, but which are engaged by the disc dome upon buckling. The knives cut the disc dome typically into quarter sections. Knife blade assemblies for reverse buckling rupture discs however add substantially to the cost of such discs and are subject to failure due to corrosive activities of the fluids within the vent system, damage during handling or simply because a mechanic forgets to install the knife assembly which in normal discs results in disc bursting pressures which are many times the rated pressures of such discs. It has, therefore, been a goal of the rupture disc industry to produce a disc of the reverse buckling type which does not include knife assemblies, but which is highly reliable.
One reverse buckling disc, which was specifically designed to rupture without use of knife blades, incorporates the concept of placing grooves, scores or etchings, especially in a criss-cross or circular patterns on concave or convex faces of a reverse buckling rupture disc dome. A disc dome of this type can be seen in U.S. Pat. No. 3,484,817 to Wood. In the Wood device, the rupture disc dome buckles, reverse and fractures along the lines of weakness produced by the grooves so as to form petals which are held to the remainder of the rupture disc assembly.
There is also a problem in some conventional systems with portions of the rupture disc being entranced with the fluid being relieved. Pieces of rupture discs can cause damage to pumps and the like if they are allowed to freely break away from the remainder of the rupture disc assembly upon rupture. Therefore, it is important that the rupture disc dome or petals of the rupture disc domain remain intact after rupture and that they remain attached to the remainder of the disc.
There has been a continuing desire in the rupture disc industry to produce new types of reverse buckling rupture discs which have properties that make them especially suitable for specific purposes, more cost efficient, and/or make the disc more reliable. In particular, new reverse buckling discs are desired which will function at lower burst pressures, and reliably open at or below the rated burst pressure if damaged, without the need for knife blades for cutting the disc on reversal, and yet which will remain attached after rupture to minimize possible damage to the system protected by the disc.
Another notable problem arises in rupture discs designed to rupture at lower pressures. Such discs become more susceptible to damage or destruction caused by induced back pressure. Vacuum pressure in the convex direction of the disc dome causes movement and fractioning at the score of a reverse rupturing disc.
The present application addresses shortcomings associated with the prior art.
The present invention is directed to an improved reverse buckling rupture disc that eliminates or at least minimizes the above-mentioned drawbacks of such prior art devices. The reverse buckling rupture disc according to the present invention includes a disc-shaped flat flange region, a concave-convex reversible disc dome and a transition region that joins the flat flange region to the disc dome region. The concave-convex reversible disc dome region has a thickness and a configuration such that the disc dome reverses when a predetermined fluid pressure is exerted on the convex side and ruptures upon reversal. The disc may have one or more deformations formed at or near the apex of the disc dome. The one or more deformations are provided to weaken the disc dome and thereby cause the disc to buckle or reverse at a lower pressure than a disc of similar thickness, diameter, crown height and material type not having the one or more deformations. This makes the disc suitable for low pressure applications.
In a second aspect of the invention, a reverse rupture disc according to the present invention includes at least one irregular transition region adjacent to the transition region and coplanar with the annular flat flange region of the disc. The disc further includes a groove which is formed along a substantial portion of the transition region of the disc. The irregular transition region of the disc dome facilities rupturing of the disc along the groove. Preferably, the groove extends around an arc of approximately 330xc2x0, but may vary as desired. Similarly, the length or region without groove may vary as required in order to retain petal after burst. The ungrooved region of the disc forms a hinge about which the reversed and ruptured disc dome remains attached to the flat flange region of the disc after rupturing. This design prevents fragmentation of the disc dome. In a preferred embodiment of the present invention, the ungrooved portion of the disc is disposed a preselected distance from the irregular transition region.
In another aspect of the present invention, a shear enhancing means aids in the rupturing of the reversed disc. The shear enhancing means cooperates with the groove to facilitate rupturing of the disc along the groove upon the reverse buckling thereof. The shear enhancing means is preferably located to cooperate with the transition region, and more specifically with an irregular transition region. The shear enhancing means may consist of a protrusion or a notch which cooperates with the irregular transition. A shear enhancing means in the shape of a notch may provide better localized stress than a protrusion given the absence of support at only one location provided by a notch compared to the existence of support at only one location provided by a protrusion. The notch focuses localized stress on the groove at the corners of the notch, allowing the rupture disc dome to flex away from the pressure at the notch. More specifically, circular, triangular, rectangular, are among an infinite number of shapes of various sizes which can be employed to further control the pressure at which the disc fractures along the groove. The shear enhancing means may be a part of (e.g., affixed to) the reverse rupturing disc, although preferably the shear enhancing means is part of a support ring, or the rupture disc holder. The support ring further includes an arcuate projection which is located adjacent to the ungrooved region of the disc. The arcuate projection provides a support surface for the disc dome region of the disc after rupturing. Alternatively, the arcuate projection may be a part of a rupture disc ring or holder.
In another aspect of the present invention a back pressure support means protects the reverse rupture disc from damage or destruction potentially caused by induced vacuum or back pressure. The above enhancements alone, and in combinations with annealing the rupture disc after forming and scoring the disc material, have enabled the design and manufacture of very low pressure reverse buckling rupture discs. However, such low pressure discs require support in the reverse direction to protect against back pressures caused, for example, by vacuum or forward to reverse pressure cyclical conditions. A back pressure support means provides additional support in the convex direction of the transition region of the disc, including support for any irregular transition region. The back pressure support means may also extend over a portion of the concave-convex disc dome, as required for functional or manufacturing purposes. The back pressure support means may be a part of the reverse rupture disc, a support ring, or the disc holder.